The present invention relates to an imaging optical apparatus that is typically used in an LED printer to allow the light from an LED array to form an image.
FIG. 8 shows the general layout of an LED printer which is generally indicated by 1A and which has the following components arranged around a photosensitive drum 10 in the clockwise direction indicated by the arrow: a charging device 12 for uniform charging of the entire surface of a photoreceptor coat 11 on the surface of the photosensitive drum 10; an LED array 13 for exposing the photoreceptor coat 11 to form an electrostatic latent image; a developing device 14 by which toner particles 14a are deposited on the electrostatic latent image to form a toner image; a transfer device 16 for transferring the toner image onto a recording sheet 15; a fixing device 17 for fixing the transferred toner image on the recording sheet 15; a cleaner 18 for removing the residual toner particles 14a on the photoreceptor coat 11; and an erase lamp 19 for removing any residual charges on the photoreceptor drum 11.
The LED array 13 consists of LED devices arranged in a two-dimensional pattern that extends along the photosensitive drum 11 in a direction parallel to the width of the recording sheet 15. In accordance with the characters, figures and other imagery to be printed, the LED devices are selectively fired to emit light L1. A rod lens array 20 is provided intermediate between the LED array 13 and the photosensitive drum 10; this consists of gradient-index rod lenses known as SELFOC lenses (the trade name of Nippon Sheet Glass Co., Ltd.) in the form of cylinders connected side to side. The rod lens array 20 condenses the light L1 into light L2 which forms an image on the photoreceptor coat 11. The image formed by the rod lens array 20 is a correct life-size image so that the image resulting from the firing of the LED devices in the LED array 13 is straightforwardly formed as the latent image.
The LED printer 1A has the problem of large overall size since not only the imaging optical apparatus consisting of the LED array 13 and the rod lens array 20 but also other image forming elements including the charging device 12 are all arranged outside the photosensitive drum 10. In order to reduce the printer size, it has been proposed that the LED array 13 and the rod lens array 20 be placed within the photosensitive drum 10 to reduce the printer size as shown in FIG. 9. In the resulting LED printer generally indicated 1B, the photosensitive drum 10 comprises a cylinder 21 that is made of a transparent or light transmissible material such as glass and which has a photoreceptor coat 11 formed on the surface. The LED array 13 emits light L1 which is condensed by the rod lens array 20 into light L2 which in turn passes through the transparent or light transmitting cylinder 21 to form an image on the photoreceptor coat 11. The image forming elements of the LED printer 1B other than the LED array 13 and the rod lens array 20, as exemplified by the charging device 12, are identical to those shown in FIG. 8 and omitted from FIG. 9.
In addition to its smaller size, the LED printer 1B has the advantage of being free from the fouling of the LED array 13 and the rod lens array 20 due to the scattering of toner particles 14a (see FIG. 8).
Nevertheless, the LED printer 1B shown in FIG. 9 has one serious problem: the thicker portion of the transparent or light transmitting cylinder 21 has a circular cross section, so when the light L1 condensed by the rod lens array 20 passes through this cylinder, it is refracted by the cylinder as if it were a concave lens. As a result, the light L2 does not form a precisely focused image and the resolution of the latent image is reduced to produce prints having only deteriorated image quality. For further details, see below.
The resolving power of a rod lens array is evaluated in terms of MTF (modulation transfer function) defined by the following equation:
MTF (%)={(imaxxe2x88x92imin))/(imax+imin)}xc3x97100
where imax and imin are a maximum and a minimum, respectively, of the light intensity on the image plane for the case where bands of light are launched into the rod lens array. The MTF as defined above is usually determined for two directions, one for the length of the rod lens array and called transverse resolution MTFx and the other for its thickness and called longitudinal resolution MTFy. The resolving power of the rod lens array is evaluated in terms of the two MTF values.
The present inventors designed a gradient-index rod lens array having the following specifications (see FIG. 10): n0=1.627; g=0.5348; h4=0.75; h6=xe2x88x921.209; h8=1.451 (n0 is the refractive index at the center of each lens, and g, h4, h6 and h8 are index gradient coefficients); angular aperture (xcex1)=20xc2x0; lens length (T)=6.89 mm; and conjugate length (C)=15.1 mm. A plurality of such lenses were combined in an array and the measurement of their MTFx and MTFy was simulated with end points A and B being assumed to present a light source and an image forming area, respectively, and the results are shown in FIG. 11. Obviously, MTFx and MTFy assume maxima at an optimum focal position (focus=0), demonstrating the high resolving power of the rod lens array.
The present inventors then modified this optical system as shown in FIG. 12 by providing a cylindrical lens 40 in the image forming area B, with its longitudinal direction being in alignment with that of the rod lens array 20 and with its concave side facing the latter. The cylindrical lens 40 was assumed to have a refractive index comparable to that of BK7 glass and adapted to have a radius of curvature (R) of 15 mm on the outer circumference, a radius of curvature (r) of 13 mm on the inner circumference, and a thickness (t) of 2 mm. The MTFx and MTFy of this cylindrical lens were also measured by simulation and the results are shown in FIG. 13. Compared to the case where the cylindrical lens 40 was absent (FIG. 11), MTFx presents a similar profile but a maximum of MTFy is way off the optimum focal position, creating a difference greater than 100 xcexcm between the optimum focal positions of MTFx and MTFy.
Most probably, this happened because the light issued from the rod lens array 20 refracted when passing through the cylindrical lens 40 and focused in an untoward point. Since the cylindrical lens 40 corresponds to the transparent or light transmitting cylinder 21 in the LED printer 1B shown in FIG. 9, a similar difference between MTFx and MTFy occurs in an actual LED printer having an imaging optical apparatus within a photosensitive drum and the latent image formed on the photoreceptor coat 11 decreases to cause eventual deterioration in the quality of printed characters and figures.
The image deterioration problem of defocusing is not limited to the LED printer but can occur in all situations where an imaging optical apparatus using a rod lens array has various transparent or light transmitting optical elements inserted into the optical path between the rod lens array and the image forming area.
The present invention is based on the review of the problems described above and its objective is to suppress the problem of reduced image resolution often encountered in imaging optical apparatus having a rod lens array with transparent or light transmitting optical elements inserted into the optical path between the rod lens array and the image forming area, as exemplified by a compact LED printer in which an imaging optical apparatus comprising an LED array and a rod lens array is placed within a photosensitive drum.
The stated object of the invention can be attained by an imaging optical apparatus comprising a gradient-index rod lens array and two transparent or light transmitting optical elements that are substantially identical in shape and imaging characteristics and which are provided in the object space and the image space in the optical path of said gradient-index rod lens array in such a way that they are symmetrical with respect to said gradient-index rod lens array.
The same object can also be attained by an imaging optical apparatus that is provided within a photosensitive drum having a photoreceptor coat formed on the outer circumference of a transparent or light transmitting cylinder and which condenses the light from an LED array with a gradient-index rod lens array and allows it to pass through the transparent or light transmitting cylinder to form an image on the photoreceptor coat, wherein a cylindrical lens that is made of substantially the same material as the transparent or light transmitting cylinder and which has substantially the same cross-sectional shape as the light transmissive portion of the transparent or light transmitting cylinder is provided such that said cylindrical lens and the light transmissive portion of the transparent or light transmitting cylinder are symmetrical with respect to the gradient-index rod lens array.
The present disclosure relates to the subject matter contained in Japanese patent application No. 2000-3638 (filed on Jan. 12, 2000), which is expressly incorporated herein by reference in its entirety.